This invention relates to a device for absorbing punching shock which occurs during the operation of a press involving the phenomenon of fracture.
In the operation of a press involving the phenomenon of fracture such as shearing, punching or blanking, there generally ensues emission of a very loud noise. To be specific, a noise due to loading impact occurs at the moment the punch collides with the workpiece. Further at the moment the workpiece is fractured, there occurs a very loud noise due to unloading impact. The reason for the emission of this loud noise is as follows. At the moment the lowered punch comes into contact with the workpiece, it begins to exert fairly large pressure to bear thereon. Owing to the consequent reaction, the frame, for example, of the press elastically deforms and accumulates strain energy. The strain energy thus built up within the frame is converted into kinetic energy in the form of vibrations at the very moment the workpiece is finally fractured under the pressure of the punch. These vibrations, upon release into the ambient air, generate the loud noise.
This means that notable abatement of this noise can be accomplished by curbing the sudden fall of the load at the moment of the fracture of the workpiece.
As a device for curbing the sudden fall of the load at the time of the fracture of the workpiece, there has been proposed a shock absorber which utilizes hydraulics (German Patent Offenlegungsschrift No. 23 50 378).
In this shock absorber, a hydraulic cylinder having a piston is provided on a press-bolster and the piston rod protruding upwardly from the hydraulic cylinder is provided on the top thereof with a thread portion which is helically engaged with a nut member. A relief valve is disposed in an oil flow path between the hydraulic cylinder and an oil tank. When a slide is lowered, the punch collides with the workpiece and fractures it and, consequently, the load of the punch is lowered suddenly. At this time, the upper end of the nut member engaged with the piston rod strikes on the press-slide, and the pressure in the hydraulic cylinder is increased to thereby compensate for the sudden fall of the load of the punch and alleviate the shock.
With this shock absorber, however, the hydraulic pressure within the cylinders is prevented from abruptly rising because of the compressibility of the hydraulic oil, leakage, etc. when the workpiece being processed happens to have a small thickness. As a result, this shock absorber has a disadvantage that it fails to provide effective elimination of the unloading impact. It is not easy to surmount this disadvantage.